In Black, I've defeated the game and wanted to check Reshiram's nature. It was Impish. I thought it was Mild so I saved. I was told that Impish is its worst nature. I released it, then checked the whole Unova and could no longer find it. So I restarted the game. And this time, I had a Timid Reshiram. I do not believe that in X & Y it will change. But who knows...